In general, bar code printers and drafting mode printers must operate at high printhead transport speeds. A printhead transport speed, U, will magnify dot placement errors caused by channel to channel variations, .DELTA.V, in the ink droplet velocity V. This may be expressed as: EQU .DELTA.x=(Ud/V.sup.2).DELTA.V (1)
where .DELTA.X is the dot placement error and d is the distance between the printhead and the printing medium. Also, for some printing applications, it is necessary to maintain a large printhead distance, d, which also magnifies dot placement errors. In general, equation (1) shows that increasing the jet velocity V will reduce .DELTA.x. It has also been observed that increasing V decreases the component of dot placement error resulting from misaim of a jet. In general therefore, when an ink jet printer is applied for in use as a bar code or draft mode printer, it is necessary to eject the ink droplets at relatively high velocities. The velocity will depend upon the print quality required i.e. the maximum dot placement error that can be tolerated. Typically, however, it will be in excess of 4.0 meters per second and less than 20 meters per second, in order to accommodate printhead transport speeds typically in excess of 10 inches per second and ranging up to 100 inches per second, relative to the print medium.
A major problem recognized by the present inventors is that when ink droplets of required high velocity for producing the quality of printing required for bar codes, for example, are ejected, the droplets tend to have relatively long ligaments trailing behind the main droplet. The ligaments reduce the quality of printing, in that they tend to break up and cause splatter printing of unwanted spurious dots on the print medium, and/or the ligaments may cause a distortion in the individual dots printed on the print medium. Accordingly, to provide necessary printing quality when using an ink jet head, for bar code and draft mode printers, it is required that the ink jet head be operated in a manner to reduce the length of the ligaments of individual ink droplets to a point where the remaining ligament does not affect the quality of printing. The present inventors also recognized the importance of insuring that the ultimate ink droplet or droplets used to print upon the print medium all have substantially the same predetermined velocity, in order to obtain close control over the printing operation.
Waveshaping techniques have been used in the prior art in order to provide control over various aspects of the operation of an ink jet printer, as will be discussed in greater detail below. For example, in Mizuno et al U.S. Pat. No. 4,491,851, a first pulse is applied to an ink jet device to initiate the ejection of an ink droplet, followed by application of a second pulse to push the "tail" of the droplet out of the nozzle and into the main droplet, thereby substantially reducing the length of the "tail" and preventing satellite droplet formation. Mizuno, and other prior art to be discussed later, do not address or even allude towards the present method for operating an ink jet printhead to avoid the problems recognized by the present inventors.